1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, heat-sensitive printing plate precursors have also become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working), by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in, e.g., EP-A 625 728, EP-A 823 327, EP-A 825 927, EP-A 864 420, EP-A 894 622 and EP-A 901 902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in, e.g., EP-A 625 728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in, e.g., EP-A 770 494, EP-A 770 495, EP-A 770 496 and EP-A 770 497. These patents disclose a method for making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying fountain solution and/or ink.
EP-A 849 091 discloses a printing plate precursor including hydrophobic thermoplastic particles having an average particle size of 40 nm to 150 nm and a polydispersity of less than 0.2.
EP-A 1 342 568 describes a method of making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying a gum solution, thereby removing non-exposed areas of the coating from the support.
WO 2006/037716 describes a method for preparing a lithographic printing plate which includes the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat and (2) developing the image-wise exposed element by applying a gum solution, thereby removing non-exposed areas of the coating from the support and characterized by an average particle size of the thermoplastic polymer particles between 40 nm and 63 nm and wherein the amount of the hydrophobic thermoplastic polymer particles is more than 70% and less than 85% by weight, relative to the image recording layer. The amount of infrared absorbing dye, hereinafter referred to as IR dye, used in this invention is preferably more than 6% by weight relative to the image recording layer.
EP-A 1 614 538 describes a negative working lithographic printing plate precursor which includes a support having a hydrophilic surface or which is provided with a hydrophilic layer and a coating provided thereon, the coating including an image-recording layer which includes hydrophobic thermoplastic polymer particles and a hydrophilic binder, characterized in that the hydrophobic thermoplastic polymer particles have an average particle size in the range from 45 nm to 63 nm, and that the amount of the hydrophobic thermoplastic polymer particles in the image-recording layer is at least 70% by weight relative to the image-recording layer. The amount of IR dye used in this invention is preferably more than 6%, and most preferably more than 8%, by weight relative to the image recording layer.
EP-A 1 614 539 and EP-A 1 614 540 describe a method of making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element disclosed in EP-A 1 614 538 and (2) developing the image-wise exposed element by applying an aqueous, alkaline solution.
EP-A 1 564 020 describes a printing plate including a hydrophilic support and provided thereon, an image formation layer containing thermoplastic resin particles in an amount from 60 to 100% by weight, the thermoplastic particles having a glass transition point (Tg) and an average particle size of from 0.01 to 2 μm, more preferably from 0.1 to 2 μm. As thermoplastic particles, polyester resins are preferred. EP 1 564 020 discloses printing plate precursors including polyester thermoplastic particles, of which the particle size is 160 nm.
EP 1 834 764 describes a negative working lithographic printing plate precursor which includes a support having a hydrophilic surface or which is provided with a hydrophilic layer and a coating provided thereon, the coating including an image-recording layer which includes hydrophobic thermoplastic polymer particles and a hydrophilic binder, characterized in that the hydrophobic thermoplastic polymer particles include a polyester and have an average particle diameter from 18 nm to 50 nm.
A first problem associated with negative-working printing plates that work according to the mechanism of heat-induced latex-coalescence is the complete removal of the non-exposed areas during the development step (i.e., clean-out). An insufficient clean-out may result in toning on the press, i.e., an undesirable increased tendency of ink-acceptance in the non-image areas. This clean-out problem tends to become worse when the particle size of the thermoplastic particles used in the printing plate precursor decreases, as mentioned in EP-A 1 614 538, EP-A 1 614 539, EP-A 1 614 540 and WO 2006/037716.
A decrease of the particle diameter of the hydrophobic thermoplastic particles in the imaging layer may, however, further increase the sensitivity of the printing plate precursor.
According to EP 1 834 764 a good clean out is obtained, even with particle sizes from 18 nm to 50 nm, when the hydrophobic thermoplastic polymer particles include a polyester. The sensitivity of the lithographic printing plate precursors including the thermoplastic polymer particles remains, however, rather low.
The rather low sensitivity of negative-working printing plates that work according to the mechanism of heat-induced latex-coalescence is a second problem to be solved. A printing plate precursor characterized by a low sensitivity needs a longer exposure time and therefore results in a lower throughput (i.e., lower number of printing plate precursors that can be exposed in a given time interval).